Shape memory alloys are practically used to utilize their peculiar functions in various fields of industries, medicine, etc. Shape memory alloys exhibiting shape memory or hyperelasticity (also called “pseudoelasticity”) phenomenon include non-ferrous alloys such as Ni—Ti alloys, Ni—Al alloys, Cu—Zn—Al alloys, Cu—Al—Ni alloys, etc., and iron alloys such as Fe—Ni—Co—Ti alloys, Fe—Mn—Si alloys, Fe—Ni—C alloys, Fe—Ni—Cr alloys, etc.
Ti—Ni alloys with excellent shape memory and hyperelasticity are practically used for medical guide wires, eyeglasses, etc. However, Ti—Ni alloys have limited applications because of poor workability and high cost.
Iron alloys advantageous in low material cost, magnetism, etc. would be expected to be used in various applications if more practical shape memory effects and hyperelasticity are obtained. However, iron-based shape memory alloys still suffer various unsolved problems. For example, Fe—Ni—Co—Ti alloys have shape memory characteristics by stress-induced transformation, but their Ms points (martensitic-transformation-starting temperatures) are as low as 200 K or lower. Fe—Ni—C alloys have poor shape memory characteristics because carbides are formed during reverse transformation. Despite better shape memory characteristics, Fe—Mn—Si alloys suffer poor cold workability and insufficient corrosion resistance, and exhibit no hyperelasticity.
JP 2000-17395 A discloses an Fe—Ni—Si shape memory alloy comprising 15-35% by weight of Ni, and 1.5-10% by weight of Si, the balance being Fe and inevitable impurities. JP 2003-268501 A discloses an Fe—Ni—Al shape memory alloy comprising 15-40% by mass of Ni, and 1.5-10% by mass of Al, the balance being Fe and inevitable impurities. These alloys contain a γ′ phase having an LI2 structure precipitated in a γ phase having an fcc structure. However, the shape memory effect and hyperelasticity of these alloys are not practically sufficient, their improvement being desired.
JP 62-170457 A discloses an iron-based shape memory alloy comprising 15-40% by weight of Mn, 1-20% by weight of Co and/or 1-20% by weight of Cr, and 15% or less by weight of at least one selected from Si, Al, Ge, Ga, Nb, V, Ti, Cu, Ni and Mn, the balance being iron. It describes that Co, Cr or Si extremely lowers a magnetic transformation point (Neel point), but does not substantially change a γ→ε martensitic transformation point. However, this alloy has substantially no hyperelasticity and a practically insufficient shape memory effect, more improvement being desired.